In digital multimeters and similar circuits, the measurement of resistance is accomplished by generating a precision constant current that is sent through an unknown resistance, and then measuring the resultant voltage drop across the resistance. Protecting the constant current source against accidental application of high voltage in the closed loop circuit of the operational amplifier has been a problem in the measurement industry. In the past, the common way to protect the current source has been to current limit in one polarity and to use a reversed biased diode in the other polarity. That system has had the limitations of the secondary breakdown of the high voltage transistor in the current limit mode, where the voltage exceeded the secondary breakdown voltage of the transistor.